I. Field of the Invention
This invention relates to a nuclear fuel element and a fuel subassembly, and more particularly, to a nuclear fuel element and a fuel subassembly suitable for use in a first breeder reactor.
II. Description of the Prior Art
Generally, a fuel body is provided with a clad so as to be prevented from a reaction with a coolant, or to prevent fission products from leaking outside. As a cladding material suitable for such a purpose, an austenitic stainless steel is exclusively used. However, irradiation of high-speed neutrons at a high temperature gives rise to a so-called swelling in austenitic stainless steel as well as in other members constituting the reactor core. The swelling is greatly attributed to the void production in metal, and therefore, an attempt to reduce the swelling by minimizing production of voids has been made at the austenitic stainless steel.
For example, an austenitic stainless steel is subjected to cold working and is increased in advance in the dislocation density in order to reduce the swelling. When, however, the stainless steel which has been cold worked is exposed to a high temperature, particularly, to temperatures of more than 700.degree. C., the dislocation density of the steel is reduced by being annealed within a relatively short period of time.
Though addition of silicon in an amount of 1.1 to 2.0% by weight to SUS 316 steel is known, the addition of such a large amount of silicon exerts a bad influence on the physical and mechanical properties of the steel. Further, the silicon also strongly acts on steel to make it into ferrite, so that austenite-forming elements such as sulphur, manganese, and nitrogen are required to be added to the steel together with silicon for the purpose of stabilizing the austenite system. Nevertheless, all these additive elements are injurious to stainless steel. For example, sulphur brings about the red shortness to and decreases the hot workability of the steel, manganese lowers the corrosion resistance, and nitrogen worsens the neutron economy and is turned into herium by a nuclear reaction to cause stainless steel to be embrittled at a high temperature.
Further, in Japanese patent application Disclosure (kokai) No. 52-130975 (laid open to the public inspection on Feb. 1, 1977), reference is made to the austenitic stainless steel containing 0.7 to 2% by weight of silicon and 0.1 to 0.5% by weight of titanium. As already mentioned above, the large amount of silicon contained in steel, particularly, the amount exceeding 1% by weight, may eventually damage the physical and mechanical properties of steel, and therefore, this is not preferable.
In the fast breeder reactor under design or construction at present, the void swelling of the stainless steel of JIS 300 series (approximately corresponding to the steel of AISI 300 series) which is used as the cladding material for the nuclear fuel is required not to exceed several percentages, for example, 6%, during the period of operation of the reactor. On the other hand, under the expected conditions of working of a liquid metal cooled fast breeder reactor (LMFBR), the void swelling of steel is considered to surely exceed 10% at the end of the prearranged period of operation, and the economical loss resulting from that is taken to be immense. Accordingly, there is a strong demand for development of the cladding material with a lower void swelling.